It's a small electronic devise designed to trans form audio into an FM signal, so that signal can be delivered to an FM radio at a set frequency?
Who Can Use a Wireless transmitter?
Anyone can use them, but they are especially useful for organizations like charities or fund raisers which may put forth 24 hour fund raising sessions.
Are They Legal?
These type of device are legal for public use, as long as they don't exceed the limits on broadcast distance set by the FCC. Usually if you keep the distance under 150 feet, your won't raise the eyebrows of the FCC.